Helical quantum Hall phase in graphene on SrTiO3

The quantum spin Hall (QSH) effect, originally observed in time reversal symmetry-protected HgTe quantum wells, is a unique phenomenon where charge carriers propagate along the (1-dimensional) edges of a 2-dimensional sample, their spin degree of freedom being locked to their direction of propagation. A similar system with counter-propagating, spin-polarized edge channels was predicted to occur in graphene in the presence of a perpendicular magnetic field, based on the specificities of Dirac fermions in the quantum Hall regime. Experimentally, the observation of such a state has been hindered for years by the presence of many-body effects stabilizing fully gapped (bulk and edges) phases.

By using substrate-screening engineering, we have recently modified the nature of electron-electron interactions in graphene and uncovered the underlying QSH effect. In this talk, I will introduce strategies to trigger a quantum spin Hall state in graphene from the quantum Hall effect. I will present magneto-transport measurements on high quality BN/graphene/BN/STO samples demonstrating the existence of a QSH state with the expected quantized resistances. The observed spin filtered, helical edge channel transport emerges at moderate magnetic fields (~ 1T), survives up to a temperature higher than 100 K over a micro long. I’ll finally present some recent scanning tunneling spectroscopy measurements that give direct access to the bulk gap related to substrate-screened exchange interaction and allows us to nail down the ground state order at the lattice scale.

L. Veyrat, C. Déprez, A. Coissard, X. Li , F. Gay, K. Watanabe, T. Taniguchi, Z. Han, B.A. Piot, H. Sellier, and B. Sacépé, Science 367, 781 (2020)

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